Semiconductor device, semiconductor laser, their manufacturing methods and etching methods

ABSTRACT

To provide a semiconductor device, such as semiconductor laser, having no need of complicated process, ensuring a high yield and mass-productivity necessary for cost reduction, and exhibiting excellent initial characteristics and reliability, nitride semiconductor layers containing a plurality of group III elements are formed on a base body surface having recess (opening) such that the nitride semiconductor layer varies in at least one of composition ratio of the group III elements, band gap energy, refractive index, electrical conductivity and specific resistance within the layer in response to the recess of the base body. In addition, by heating the structure in an atmosphere containing hydrogen and using a layer containing Al as an etching stop layer, controllability and production yield can be improved without influences from fluctuation in etching depth, or the like. Further, etching and re-growth can be conducted consecutively to provide an inexpensive process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the benefit of priorityunder 35 USC §120 from U.S. Ser. No. 10/816,929, filed Apr. 5, 2004,which is a division of U.S. Pat. No. 6,741,623, issued May 25, 2004, andis based upon and claims the benefit of priority under 35 USC §119 fromthe prior Japanese Patent Application No. 2000-199217, filed on Jun. 30,2000, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor device, semiconductor laser,their manufacturing methods and etching methods, and more particularly,to a semiconductor device using nitride semiconductors and requiringselective processing for a current confining structure, for example. Theinvention also relates to, in particular, a high-performancesemiconductor laser controlled in current confinement and transversemode, and a manufacturing method thereof.

2. Related Background Art

Group III nitride semiconductor materials, which enable realization ofmaximum band gap energies among III-V compound semiconductor materialsand can make hetero junctions, are remarked as hopeful materials ofsemiconductor lasers and light emitting diodes for emission ofshort-wavelengths, or high-speed, high-output electronic devices. Fortypical devices using group III nitride semiconductor materials,thin-film forming techniques using metal-organic chemical vapordeposition (MOCVD) and epitaxial growth such as molecular beam epitaxy(MBE) are often used. In case of electronic devices having heterojunctions, such as semiconductor lasers, light emitting diodes, etc.,those thin-film growth techniques are used to form a plurality ofnitride mixed crystal thin film layers of group III elements differentin composition ratio and use differences in band gap energy among theselayers to confine light or electrons.

Such nitride mixed crystal thin film layers are typically formed onvarious base bodies. Upon their epitaxial growth, composition of a mixedcrystal thin film layer grown under a fixed growth condition, i.e.composition of a mixed crystal thin film layer grown in a single processof crystal growth, was usually uniform over the entire surface. Usually,therefore, physical properties of the mixed crystal, such as bandgapenergy, refractive index, conductivity, specific resistance, and soon,were uniform over the entire surface of the thin film layer formed onthe base body. Although there is a report about generation ofnon-uniformity, which reports fine deposition regions different incomposition are formed in an InGaN thin film, from a macro-scaleviewpoint, those physical properties are not but ones that should beregarded to be substantially uniform throughout the region having formedthe thin film layer on the base body. Additionally, there is a reportalso regarding group III nitride semiconductors that a so-called“superlattice”, made by periodically forming a plurality of very thinfilms of a thickness several to tens of times an atomic layer, has beenmade. Here again, those physical properties of the entirety of thesuperlattice layer are not but ones that should be regarded to besubstantially uniform over the entire surface of the thin film layer onthe base body.

Thus it has been considered that composition and physical properties ofany nitride mixed crystal thin film layer made in a single process ofcrystal growth are inevitably uniform over the entire surface of thethin film layer. Therefore, in order to intentionally vary physicalproperties of the nitride mixed crystal thin film layer in the surfacedirection of the base body in a semiconductor laser, light emittingdiode, electronic device, or the like, it has been necessary to carryout a plurality of epitaxial growth steps and etching steps, as well asadditional complicated steps for positional alignment.

FIG. 12 is a cross-sectional view that shows configuration of aconventional semiconductor laser using nitride mixed crystal thin filmlayers. The laser of FIG. 12 includes an n-type GaN contact layer 912,n-type AlGaN cladding layer 914, InGaN quantum well active layer 916,p-type AlGaN cladding layer 918, and p-type GaN contact layer 920, whichare thin film layers uniform in the surface direction, formed on asurface of a sapphire substrate 910 as a base body. The p-type claddinglayer 918 is ridge-shaped to enhance the optical guide efficiency. Forcurrent confinement, the laser further includes an insulating film 930having an opening above the ridge of the p-type cladding layer 918, andthrough this opening, a p-side electrode 950 is formed. Connected to then-type contact layer 912 is an n-side electrode 940.

The semiconductor laser shown in FIG. 12 needs a complicated processincluding selective etching of the p-type cladding layer and others formaking the waveguide, current confinement or electrode contact,formation of the insulating film 930, formation of the p-side electrode950 and n-side electrode 940, and so on. It therefore involves theproblems that the production yield is low and the productivity necessaryfor reducing the cost is low. Additionally, there is the problem thatdamage to crystals during etching and other process deteriorate theinitial characteristics and reliability of the device.

As reviewed above, conventional techniques could only obtain uniformphysical properties of any nitride mixed crystal thin film layer formedon a base body. So, for fabricating a semiconductor laser, lightemitting diode, electronic device, or the like, the conventionaltechniques had to use processing techniques requiring a plurality ofepitaxial growth and complicated positional alignment in order to varyphysical properties such as band gap energy, refractive index,conductivity and specific resistance along the horizontal surface of thebase body and for hereby eliciting functions. And this invited theproblems that the production yield was low, productivity necessary forreducing the cost was low, or damage to crystals during the use of thoseprocessing techniques deteriorated the initial properties andreliability of the device.

On the other hand, apart from those problems, semiconductor lasers usingnitride semiconductors had need of a technique that could reliably stopetching at a predetermined etching depth.

That is, blue semiconductor lasers using nitride semiconductors likeInAlGaN, which have short wavelengths and can therefore make small beamdiameters, are recently looked for as light sources for high-densityinformation processing with optical disks, for example. For applicationto optical disc systems, for example, it is necessary to convergeemanating beams of semiconductor lasers to minimum spots, and basictransverse mode oscillation is indispensable.

A number of devices with conventional ridge structures have beenreported as nitride semiconductor lasers. Ridge structures, however, arecharacterized in that the difference in effective refractive indexbetween the ridge portion important for transverse mode control and theexterior of the ridge largely depends on the etching depth. For years,dry etching represented by reactive ion etching (RIE) and reactive ionbeam etching (RIBE) has been widely used in the etching process formaking the ridge. However, regarding dry etching of nitridesemiconductors, there is not yet established any technique, such asselective etching method, capable of stopping the etching at a targetetching depth, and the etching depth is controlled by adjusting theetching time or by monitoring the progress of the etching through alaser interferometer, for example. With these control methods of theetching depth, however, it is difficult to stop the etching at theinterface with the underlying layer or stop the etching so as to keep adesired thickness over the entire wafer surface, and sufficient controlof the etching depth is impossible.

Thus, the conventional etching techniques cannot control the etchingdepth sufficiently. Additionally, since ridge structures are affected bythe thickness profile of the film by crystal growth, etching depthprofile, and so forth, it was difficult to fabricate devices controlledin basic transverse mode with a good yield. That is, in InAlGaNsemiconductor lasers having conventional ridge structures, theirstructures themselves invite large influences to their characteristicsfrom process accuracy and unevenness. Therefore, it was difficult tofabricate lasers for continuous oscillation in the basic transverse modewith a good yield.

SUMMARY OF THE INVENTION

The invention has been made under acknowledgement of the above-mentionedvarious problems.

It is therefore the first object of the invention to provide asemiconductor laser not requiring complicated processes, having a highyield and a productivity necessary for lowering the cost, and havingexcellent initial properties and reliability by varying the compositionand physical properties of each nitride mixed crystal thin film layerformed in a single process of crystal growth within the film.

The second object of the invention is to provide a nitride semiconductorlaser having a transverse mode control structure excellent incontrollability of the etching depth, not affected by deterioration ofthe device properties due to etching damage, and excellent inproductivity such as production yield.

The third object of the invention is to provide a semiconductor devicenot requiring complicated processes, having a high yield and aproductivity necessary for lowering the cost, and having excellentinitial properties and reliability by varying the composition andphysical properties of each nitride mixed crystal thin film layer formedin a single process of crystal growth within the film.

The fourth object of the invention is to provide a manufacturing methodof a laser device not requiring complicated processes, and having a highyield and a productivity necessary for lowering the cost by employing atechnique that can made a nitride mixed crystal thin film layerdifferent in composition and physical properties within the film in asingle process of crystal growth.

The fifth object of the invention is to provide a selective etchingtechnique of a nitride semiconductor, which can make an excellenttransverse mode control structure.

With those problems taken into account, according to an aspect of theinvention there is provided a semiconductor laser comprising:

-   -   a substrate;    -   a nitride semiconductor layer made of a nitride semiconductor on        said substrate and having a stripe-shaped opening;    -   a buried layer burying said stripe-shaped opening and made of a        nitride semiconductor containing at least two kinds of group III        elements, said buried layer including a first portion lying in        and above said opening and a second portion lying on said        nitride semiconductor layer, said first portion of said buried        layer being different from said second portion of said buried        layer in composition ratio of said at least two kinds of group        III elements; and    -   an active layer formed on said buried layer.

According to another aspect of the invention, there is provided asemiconductor laser comprising:

-   -   a substrate;    -   a first cladding layer of a first conduction type made of a        nitride semiconductor of a first conduction type on said        substrate;    -   a current blocking layer formed on said first cladding layer of        the first conduction type and having a stripe-shaped opening        which partly exposes said first cladding layer of the first        conduction type, said current blocking layer having a first        layer of a nitride semiconductor formed adjacent to said first        cladding layer of the first conduction type and a second layer        of a nitride semiconductor formed on said first layer, said        first layer being made of a material more likely etched than        said second layer and said first cladding layer of the first        conduction type;    -   a second cladding layer of the first conduction type made of a        nitride semiconductor of the first conduction type lying in and        above said opening and on said current blocking layer so as to        bury said stripe-shaped opening; and    -   an active layer formed on said second cladding layer of the        first conduction type.

According to a further aspect of the invention, there is provided asemiconductor device comprising:

-   -   a base body having at least one recess; and    -   a buried layer made of a nitride semiconductor containing at        least two kinds of group III elements lying on said base body to        bury said recess with a part thereof, said buried layer        including a first portion lying in and above said recess and a        second portion lying outside of said recess wherein, said buried        layer varying in composition ratio of said at least two kinds of        group III elements between said first portion and said second        portion.

According to a still further aspect of the invention, there is provideda semiconductor laser manufacturing method comprising:

-   -   forming a nitride semiconductor layer by a crystal growth for        crystallographically growing a nitride semiconductor on a        substrate;    -   selectively etching said nitride semiconductor to form a        stripe-shaped opening;    -   forming a buried layer by crystallographically growing a nitride        semiconductor containing at least two kinds of group III        elements in and above said opening and on said nitride        semiconductor layer; and    -   for forming an active layer of a nitride semiconductor on said        buried layer.

According to a yet further aspect of the invention, there is provided asemiconductor laser manufacturing method comprising:

-   -   sequentially forming on a substrate an etching stop layer of a        first conduction type nitride semiconductor, an etching layer of        a nitride semiconductor and an etching mask layer of a second        conduction type nitride semiconductor, said nitride        semiconductor of said etching layer being more likely etched        than those of said etching mask layer and said etching stop        layer;    -   partly etching said etching mask layer to form a stripe-shaped        first opening to expose a part of said etching layer in said        first opening;    -   heating said etching layer in a mixed atmosphere containing        hydrogen and at least one of nitrogen, ammonium, helium, argon,        xenon and neon, or a mixed atmosphere of nitrogen and ammonium,        or a hydrogen atmosphere to etch said etching layer exposed in        said first opening and thereby form a stripe-shaped second        opening to expose a part of said etching stop layer;    -   burying said first opening and said second opening with a buried        layer of a first conduction type nitride semiconductor; and    -   forming an active layer on said buried layer.

According to a yet further aspect of the invention, there is provided anetching method for selectively etching a first nitride semiconductorlayer relative to a second nitride semiconductor layer comprising:

-   -   etching said first nitride semiconductor layer by heating it in        a mixed atmosphere containing hydrogen and at least one of        nitrogen, ammonium, helium, argon, xenon and neon; or a mixed        atmosphere of nitrogen and ammonium; or and a hydrogen        atmosphere.

As used in this specification, a “nitride semiconductor” pertains to anysemiconductor having any composition in which composition ratios x, yand z vary within their respective ranges in the chemical formulaB_(1-x-y-z)In_(x)Al_(y)Ga_(z)N (x≦1, y≦1, z≦1, x+y+z≦1). For example,InGaN (x=0.4, y=0, z=0.6) is also a kind of “nitride semiconductors” asused herein. Further, the “nitride semiconductors” should be construedto involve those in which part of the group V element, N (nitrogen), hasbeen replaced by As (arsenic) or P (phosphorus). In this case, any suchnitrogen semiconductor contain one of those three group III elements(In, Al, Ga) and at least N (nitrogen) as the group V element.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram illustrating a cross-sectional structureof a central part of any semiconductor laser according to an embodimentof the invention;

FIG. 2 presents cross-sectional views schematically showing a processfor growing a cladding layer 108 on a current blocking layer 106;

FIG. 3 is a cross-sectional view showing in an enlarged scale a centralpart including the current blocking layer 106 in the semiconductor laserof FIG. 1;

FIG. 4 presents schematic cross-sectional views showing an etchingprocess according to the second embodiment of the invention;

FIG. 5A is a schematic cross-sectional view that shows a central part ofa semiconductor laser as the first example of the invention;

FIG. 5B is a schematic, enlarged, cross-sectional view of a central partof a semiconductor laser as the first example of the invention;

FIG. 6 is a diagram that shows a cross-sectional structure of a nitridesemiconductor laser according to the second example of the invention;

FIG. 7 presents cross-sectional views showing a manufacturing process ina manufacturing method of a semiconductor laser taken as the thirdexample of the invention;

FIG. 8 is a schematic diagram that shows a cross-sectional structure ofthe semiconductor laser taken as the third example of the invention;

FIG. 9 is a schematic diagram that shows a cross-sectional structure ofa semiconductor laser taken as the fourth example of the invention;

FIG. 10 is a schematic diagram that shows a cross-sectional structure ofa semiconductor laser taken as the fifth example of the invention;

FIG. 11 presents schematic cross-sectional views illustrating amanufacturing process in a manufacturing method of the semiconductorlaser taken as the fifth example of the invention; and

FIG. 12 is a cross-sectional view showing a representative structure ofconventional semiconductor lasers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will now be explained below with referenceto the drawings.

First shown is the basic structure of semiconductor devices according toembodiments of the invention.

FIG. 1 is a schematic diagram that shows a cross-sectional structure ofa central part of any semiconductor laser device according toembodiments of the invention. That is, FIG. 1 shows a cross-sectionalstructure of an edge-emitting semiconductor laser obtained by using theinvention, which is viewed from its emanation edge, and shows an n-typebuffer layer 102, first n-type cladding layer 103, current blockinglayer 106, p-type contact layer 108, active layer 109, p-type claddinglayer 110 and p-type contact layer 111 sequentially formed on an n-typesubstrate 101. The current blocking layer 106 has a stripe-shapedopening S extending substantially vertically to the drawing sheet, andthis opening is buried with the second cladding layer 108. Formed on thep-type contact layer 111 is the p-side electrode 112 whereas the n-sideelectrode is formed on the bottom surface of the n-type substrate 101.Those layers from the buffer layer 102 to the p-type contact layer 111may be made of nitride semiconductors.

Next explained are roles of the respective layers. The buffer layer 102functions as a buffer for improving crystalline properties of therespective layer formed on the substrate 101. The cladding layers 103,108, 110 each have a larger band gap than that of the active layer 109and functions to confine carriers and light and thereby bring aboutlaser oscillation. As explained later in greater detail, the activelayer 109 also functions as an etching stopper layer during etching ofthe current blocking layer 106. The current blocking layer 106 serves asa block layer for confining a current injected from outside into thestripe-shaped core region. Additionally, as explained later in greaterdetail, the current blocking layer 106, having an stripe-shaped openingS, also functions to vary the second n-type cladding layer 108 grownthereon in composition in the surface direction. The active layer 109functions to recombine injected carriers thereby to bring about emissionof light corresponding to its band gap. The p-type contact layer 111functions to reduce the contact resistance with the electrode formedthereon. The invention, however, is not restrictive to the illustratedconduction types, but the conduction types of respective layers may beinverted. Further, the current blocking layer may be positioned on theactive layer.

With reference to the basic construction explained above, embodiments ofthe invention will be explained below.

First Embodiment

First referring to FIG. 1, the first embodiment of the invention will beexplained. One of features of this embodiment is that the second n-typecladding layer 108 varies in composition between the portion in andabove the stripe-shaped opening S and the portion on current blockinglayer 106. That is, in this embodiment, the second n-type cladding layer108 is made of a nitride semiconductor containing two or more kinds ofgroup III elements. Moreover, composition ratios of these group IIIelements are different between the portion in and above thestripe-shaped opening S and the portion on current blocking layer 106.

For example, let the cladding layer 108 be made of AlGaN. Then, the Alcomposition of the cladding layer 108 is lower in and above thestripe-shaped opening S than on current blocking layer 106. Therefore, aprofile of refractive indices corresponding to the waveguide stripe isproduced within the cladding layer 108. That is, refractive index of theAlGaN cladding layer 108 formed inside and above the stripe-shapedopening S is higher than formed on current blocking layer 106.Therefore, the function of a waveguide for guiding light can be built inthe cladding layer 108. As a result, lateral transverse mode of lightpropagating in the semiconductor laser can be efficiently confined, andthe oscillation property can be improved significantly.

The in-plane distribution of composition of the cladding layer 108 canbe produced in a single process of crystal growth by using thestripe-shaped opening S of current blocking layer 106 as explainedbelow.

FIG. 2 presents cross-sectional views schematically showing a processfor growing the cladding layer 108 on the current blocking layer 106.Once the AlGaN cladding layer 108 is grown on the current blocking layer106 having a stripe-shaped opening S by any of various depositionmethods such as MOCVD or MBE, crystal growth initially starts whilereflecting the shape of the opening S as shown in FIG. 2(a). However, asthe crystal growth progresses, the opening S is gradually buried asshown in FIG. 2(b), and it is formed to a certain thickness, the openingS is fully buried, and the surface of the cladding layer 108 exhibits asubstantially flat plane as shown in FIG. 2(c). Such flattening of thesurface of the cladding layer 108 occurs because deposition particlesflying onto the current blocking layer 106 migrate (move) into theopening S as shown by arrows M in FIG. 2. Its mechanism is presumed suchthat, in the group III source material or its decomposition product(including the group III element itself as well) supplied onto the basebody by vapor phase deposition, for example, there is a difference incoupling force with the material covering the base body surface betweenAl (aluminum) and the other element (here is used Ga (gallium)), forexample. That is, on the surface of the base body, Al or itsdecomposition product is unlikely to migrate as compared with Ga and itsdecomposition product. As a result, Ga migrates more than Al to openingS, and the Al composition decreases in and above the opening S.

Though, when the crystal growth progresses further after the opening Sis fully buried, Ga does not migrate. Therefore, when AlGaN claddinglayer 108 becomes thick, the Al composition of the surface side of thecladding layer 108 becomes uniform. As known from this specification,“AlGaN cladding layer 108 lying above the stripe-shape opening S” doesnot involve the portion of the above portion of the stripe-shape openingS where Ga does not migrate.

As explained above, according to the instant embodiment, nitridesemiconductor layers each varying in composition and refractive indexwithin the film can be made in a single process of crystal growth. Thatis, by making the stripe-shaped opening S in the current blocking layer106 and making the cladding layer 108 to bury the opening recess, it spossible to vary compositions of group III elements in the claddinglayer to produce a profile of refractive indices, and thereby, the lightconfinement efficiency can be improved. In order to enhance this effect,it is desirable that group III elements forming the cladding layer 108are largely different in migration rate, and hence likely to produce adifference in refractive index. When fabricating a nitride semiconductorlaser, compositions can be varied easily by employing a materialcontaining Al and another group III element as the nitride semiconductorforming the cladding layer 108.

In addition, according to the instant embodiment, since compositions ofgrown layers can be varied in accordance with recesss and projectionsalong the base body surface, it is possible to not only produce aprofile of refractive indices but also spatially change various physicalproperties including band gap energy, conductivity, specific resistanceand so on. Therefore, not limited to semiconductor lasers, the inventionis applicable to various kinds of optical devices and electronicdevices.

Second Embodiment

Next explained is the second embodiment of the invention. Thisembodiment is directed to a structure based on the semiconductor lasershown in FIG. 1, for example, in which the stripe opening S of thecurrent blocking layer 106 can be made reliably and easily, and anetching process thereof.

FIG. 3 is a cross-sectional view showing in an enlarged scale a centralpart including the current blocking layer 106 in the semiconductor laserof FIG. 1. That is, in the instant embodiment, the current blockinglayer 108 is made by forming an etching layer 104 and an etching masklayer 105. The etching layer 104 is made of a material exhibiting ahigher etching rate than the etching mask layer 105 under apredetermined etching condition.

For example, in case a nitride semiconductor is used, Al composition ofthe etching layer 104 is adjusted to be lower than the Al composition ofthe etching mask layer 105.

The first cladding layer 103 under the current blocking layer 106functions as an etching stop layer. That is, a material is used for thefirst cladding layer 103, which exhibits a lower etching rate than theetching layer 104 under a predetermined etching condition. For example,if a nitride semiconductor is used, Al composition of the first claddinglayer 103 should be higher than that of the etching layer 104.

As explained above, the instant embodiment uses the three-layeredstructure made up of the etching stop layer (first cladding layer) 103,etching layer 104 and etching mask layer 105.

FIG. 4 presents schematic cross-sectional views showing an etchingprocess according to the second embodiment.

First as shown in FIG. 4(a), the etching stop layer 103, etching layer104 and etching mask layer 105 are formed sequentially.

Next as shown in FIG. 4(b), a mask 800 having a predetermined opening Ois formed on the etching mask layer 105. Material of the mask 800 may beadequately chosen from various materials including resist and siliconoxide, for example.

Next as shown in FIG. 4(c), the etching mask layer 105 is selectivelyetched by using a first etching method to make a first opening. Thefirst etching method can be executed under a condition etching theetching mask 105 alone and not etching the etching layer 104.

Alternatively, the first etching method may be conducted under acondition etching the etching layer 104 as well. In this case, however,the etching has to be stopped before the etching layer 104 exposed inthe opening O is fully removed by etching.

In case a nitride semiconductor is to be etched, usable as the firstetching method is any of, for example, dry etching such as CDE (chemicaldry etching), RIE (reactive ion etching) or ion milling, and wet etchingusing an etching liquid like KOH.

Next as shown in FIG. 4(d), the mask 800 is removed. This step, however,may be executed after the step shown in FIG. 4(e).

Next as shown in FIG. 4(e), using the etching mask layer 105 as a mask,the etching layer 104 is etched by a second etching method to form asecond opening. The second opening and the above-mentioned first openingmake up the opening S in FIG. 4(e). Hereinbelow, the first opening,second opening and opening S will be often called the openingcollectively without distinguishing them from each other. The secondetching method is conducted under a condition ensuring a high etchingrate for the etching layer 104 and a low etching rate for the etchingstop layer (first cladding layer) 103. Thus it is possible to reliablystop the etching to obtain the configuration with the etching layer 104being removed and the etching stopper layer 103 being exposed.

More specifically, taking a nitride semiconductor as the object to beetched, a vapor phase etching method originally developed by theInventor can be used as the second etching method. For example, let anexample be taken, in which the etching stop layer 103 is made of AlGaN,the etching layer 104 of GaN (or AlGaN with a low Al composition) andthe etching mask layer 105 of AlGaN. In this case, if the temperature israised approximately to 1000° C. in an atmosphere containing hydrogen,the etching layer 104 is etched, but almost no etching occurs in theetching stop layer 103 and the etching mask layer 105. That is, it ispossible to selectively etch the etching layer 104 alone and reliablystop the etching at he etching stop layer 103. This means that apredetermined configuration can be obtained by etching without invitingover-etching or under-etching. In addition, when using the vapor phaseetching method, the crystal is not damaged by the etching gas or plasma.

Further, it became clear through researches by the Inventor that usableatmospheres ensuring the effect of vapor phase etching to nitridesemiconductors include a mixed atmosphere combining one of nitrogen,ammonium, helium, argon and xenon with hydrogen, a mixed atmospherecombining two of those elements with hydrogen, a mixed atmospherecontaining nitrogen and ammonium, and a hydrogen atmosphere.

One of features of these atmospheres is that they do not chemicallyreact so hard with nitride semiconductors, unlike so-called etchinggases heretofore used in conventional dry etching such as CDE and RIE.That is, the vapor phase etching method according to the invention has aunique feature in not using corrosive reactive gases having been usedheretofore.

As explained above, according to the instant embodiment, by employingthe three-layered structure of he etching stop layer 103, etching layer104 and etching mask layer 105, and properly adjusting the etchingmethod, the etching can be stooped reliably and easily at the etchingstop layer 103, and a desired post-etching structure can be obtained.Therefore, various structures like current confinement structures orlight confinement structures can be made precisely with a goodreproducibility.

EXAMPLES

Regarding the first and second embodiments explained above, moredetailed explanation will be made below by way of specific examples.

First Example

A specific example of the first embodiment explained above will be firstexplained below as the first example of the invention.

FIG. 5A is a schematic cross-sectional view that shows a central part ofa semiconductor laser as the first example of the invention. In FIG. 5A,components identical or equivalent to those already explained withreference to FIG. 1 through FIG. 4 are labeled with common referencenumerals. That is, the n-type GaN buffer layer 102 is formed on then-type GaN substrate 101, and the first n-type AlGaN cladding layer 103is formed on the n-type GaN buffer layer 102. Formed on the first n-typeAlGaN cladding layer 103 is the current blocking layer 106 made up ofthe GaN layer 104 and the p-type AlGaN layer 105, and formed in thecurrent blocking layer 106 is the stripe-shaped opening S that reachesthe first n-type AlGaN cladding layer 103. These components heretoforementioned form the base body 107. Aluminum composition of the firstn-type AlGaN cladding layer 103 is preferably 0.30 or lower. If thealuminum composition exceeds 0.30, resistance of the layer substantiallyincreases, and this is undesirable. In this example, aluminumcomposition of the first n-type AlGaN cladding layer 103 is 0.07.Aluminum composition of the p-type AlGaN layer 105 forming the currentblocking layer 106 is preferably 0.05 or higher, and equal to or higherthan the aluminum composition of the first n-type AlGaN cladding layer103. Such composition ensures that the p-type AlGaN layer 105effectively functions as a current blocking layer. In this example,aluminum composition of the p-type AlGaN layer 105 is 0.07, which is thesame as that of the first n-type AlGaN cladding layer 103. The p-typeAlGaN layer 105 may be a p-type AlN layer as well.

Formed on the base body 107 having the stripe-shaped opening S is thesecond n-type AlGaN cladding layer 108. Further, the active layer 109having a quantum well structure containing InGaN is formed on the secondn-type AlGaN cladding layer 108, and the p-type AlGaN cladding layer 110and the p-type GaN contact layer 111 are formed on the active layer 109.The p-side electrode 112 is made of Pt (platinum)/Au (gold), forexample, on the p-type GaN contact layer 111, and the n-side electrode113 is made of Ti (titanium)/Al (aluminum) or Ti (titanium)/Au (gold) onthe other surface of the n-type GaN substrate opposite from the activelayer 109.

In the structure shown in FIG. 5A, the p-type AlGaN cladding layer 108lies on the base body 107 having the stripe-shaped recess (opening) S,and Al composition of the p-type AlGaN layer 108 is lower in the andabove the recess S than outside of the recess S. Therefore, horizontaltransverse mode of light propagating in the semiconductor laser can beconfined in the stripe channel portion.

Next explained is a manufacturing method of the semiconductor laserdevice of FIG. 5A.

First prepared is the Si-doped n-type (first conduction type) GaNsubstrate 101 having (0001) plane as its one surface, and grown thereonis the Si-doped n-type GaN buffer layer 102 by metal-organic chemicalvapor deposition (MOCVD). Next grown is the Si-doped n-typeAl_(0.07)Ga_(0.93)N cladding layer 103 to the thickness of 0.6 μm. Nextformed the current blocking layer 106 composed of the undoped GaN layer,0.2 μm thick, and the Mg-doped p-type (second conduction type)Al_(0.07)Ga_(0.93)N, 0.2 μm thick.

After that, the wafer is removed from the growth apparatus and undergoesmasking and selective etching by reactive ion etching (RIE) to make thestripe-shaped opening, 3 μm wide. In this etching process, a thickness,for example, 0.3 μm, is targeted, such that the channel penetrates atleast the Mg-doped p-type AlGaN layer 105 and reaches the undoped GaNlayer 104. Regarding the direction of the stripe, it should extendnormally to the plane (1-100) of the GaN substrate 101.

After that, the wafer is returned into the growth apparatus and heatedto a growth temperature in a mixed gas atmosphere containing hydrogen,nitrogen and ammonium. Then undoped GaN layer 104 exposed in thestripe-shaped opening is etched by etching progresses due tore-vaporization, and the n-type AlGaN cladding layer 103 is exposed.Through those steps, the base body 107 having recess on the surface isobtained.

Subsequently, the Si-doped n-type AlGaN cladding layer 108 is formed byMOCVD. In this step, by adjusting its Al composition to beAl_(0.1)Ga_(0.9)N in the portion on the current blocking layer 106 andforming it to 0.2 μm. It resulted that Al composition in the portion ofthe opening S is Al_(0.07)Ga_(0.93)N. The thickness of AlGaN claddinglayer 108 lying in and above the portion of opening S is 0.6 um, whilerendering the surface of AlGaN cladding layer substantially flat.

Next formed on the n-type AlGaN second cladding layer 108 is the activelayer 109 having: 0.1 μm thick undoped GaN layer, 10 nm thick Si-dopedIn_(0.02)Ga_(0.98) N barrier layer, the quantum well structure regionmade by repeating two cycles of 4 nm thick undoped In_(0.08)Ga_(0.92)Nand 10 nm thick Si-doped In_(0.02)Ga_(0.98)N barrier layers, cap layermade up of 20 nm thick undoped In_(0.2)Ga_(0.98)N and 20 nm thickMg-doped Al_(00.2)Ga_(0.8)N, 0.1 μm thick Mg-doped GaN layer. Furtherformed on the active layer 109 are the 0.6 μm thick Mg-doped p-typeAl_(0.07)Ga_(0.93)N cladding layer 110 and the 0.2 μm thick Mg-dopedp-type GaN contact layer 111.

After that, the wafer with grown crystals on one surface of n-type GaNsubstrate is removed from the growth apparatus, and the p-side electrode112 made of Pt/Au is formed on the p-type GaN contact layer 111.Further, the other surface of n-type GaN substrate 101 is polished untilobtaining a thickness around 80 μm, and the n-side electrode 113 made ofTi/Au is formed on the other surface of the n-type GaN substrate 101.

After that, the wafer having the structure is cleaved into bars suchthat a plane normal to the stripe direction of the laser, i.e. the(1-100) plane of the GaN substrate 101, comprises each edge of each bar.Edges of each cleaved bar undergoes adequate edge coating, andthereafter dicing and braking, thereby to separate them in form ofdiscrete chips. Each chipped laser device will be attached on a heatsink, with its p-side electrode 112 adjacent to the heat sink, bywelding using an AuSn (gold-tin) solder, for example.

The semiconductor laser obtained through those steps not only exhibiteda good current confinement performance, but also exhibited goodproperties including: maintaining basic modes in both vertical andhorizontal transverse modes even under high optical outputs beyond 50 mWused for writing onto optical disks, permitting only a sufficiently lowoptical output by spontaneous radiation near the oscillation thresholdcurrent, and decreasing noise even during low-power reading.

That is, by having nitride mixed crystal thin film layers formed on abase body to locally vary in physical properties, which was difficultwith conventional techniques, the method could actually manufacture asemiconductor laser device, which has a lower Al composition in andabove the recess S than outside of the recess S, productivity necessaryfor reducing the cost, and excellent initial characteristics.

The essence of the mechanism enabling fabrication of such ahigh-performance device lies in that the AlGaN layer 108 could be madeon the base body 107 having the stripe-shaped recess S to have a higherAl composition in and above the recess S than outside of the recess Sand that the AlGaN layer 108 could be formed thick to represent asubstantially flat surface. As a result, refractive index was higher inand above the recess S portion than that outside of the recess Sportion, and the function of a waveguide for guiding light could bebuilt in.

The reason why this structure can be made is presumably that, amonggroup III source materials and their decomposition products (includinggroup III elements themselves) supplied onto the base body by vaporphase deposition, there is a difference in coupling force with amaterial covering the base body between those containing Al and thosecontaining any other (Ga, in this example).

That is, it is considered that, at the temperature from about 1000° C.to about 1100° C. where AlGaN layer 108 is formed, comparing with Gasource materials and their decomposition products, Al source materialsor their decomposition products have larger coupling forces to base bodysurface, and smaller liability to migration along the base body surface.Therefore, materials with a lower Al composition is formed in and abovethe recess S. It is also assumed that the assumed mechanism alsocontributes to acceleration of growth rate in and above the recess S.These effects are weakened as the surface becomes more flatter togetherwith the progress of the crystal growth on the base body surface, andthis is considered to result in flattening the surface of the AlGaNlayer 108.

In the example shown above, the n-type AlGaN cladding layer contains Aland Ga mixed in bulk; however, it may be a superlattice AlGaN layer madeby repeating some cycles of GaN layers and AlGaN layers.

FIG. 5B is a schematic, enlarged, cross-sectional view of a central partof a semiconductor laser having such a superlattice AlGaN layer. Thatis, the cladding layer 108 has a structure alternately forming AlGaNbarrier layers 108A and GaN well layers 108B. Even when the opening S isburied with the superlattice structure, composition of group IIIelements can be varied similarly. That is, by changing composition andthickness of each thin-film layer forming the superlattice betweenportion in and above the recess S and portion outside of the recess S, awaveguide structure equivalent to that of the bulk layer can be made.More specifically, Al composition of the AlGaN barrier layer 108A islower in and above the recess S than outside of recess S. As a result,mean refractive index of the superlattice cladding layer 108 is higherin and above the recess S than outside of the recess S, and lightconfinement efficiency can be raised there.

FIG. 5B shows the superlattice structure in an abridged form forsimplicity. Actually, however, barrier layers 108A and well layers 108Bare thinner than illustrated, and the number of layers may be more thanillustrated, respectively.

In the example shown above, the current blocking layer 106 is made up ofthe undoped high-resistance first layer 104 and the Mg-doped p-typesecond layer 105. However, only if the current blocking layer 106 hasthe function of blocking part of the current injected into the activelayer 109, the first layer 104 and the second layer 105 may be changedin conduction type. For example, while maintaining the first layer 104as it is, the second layer 105 may be changed to a high-resistancelayer, or the first layer 104 may be changed to a p-type layer whilemaintaining the second layer 105 as it is. Alternatively, the firstlayer 104 may be changed to a p-type layer, and the second layer 105 toa high-resistance layer.

Furthermore, the above example has been explained as using conductiveGaN as the substrate. However, insulating substrate such as sapphire canbe used, then p-side and n-side electrodes is formed on same said of theinsulating substrate. And having nitride mixed crystal thin film layerson the base body varied in physical properties within the horizontalplane of the base body, which was difficult conventionally, it ispossible to provide a semiconductor laser device having a high yield,productivity necessary for cost reduction, excellent initialcharacteristics and reliability without using complicated processes.

The above example has been explained, taking a semiconductor laser whichincludes a stripe-shaped recess as its base body and nitridesemiconductor layers that are formed on the base body and contain aplurality of group III elements including at least Al. However, therecess need not be stripe-shaped, and a similar confining structure madeby a circular or rectangular opening is applicable to a surface emittinglaser. Here again, by having nitride mixed crystal thin film layers onthe base body varied in physical properties within the horizontal planeof the base body, which was difficult conventionally, it is possible toprovide a semiconductor laser device having a high yield, productivitynecessary for cost reduction, excellent initial characteristics andreliability without using complicated processes.

Moreover, although the above example has been explained as applying itto a semiconductor laser, it can be applied to light emitting diodes,optical waveguides, optical switches, and so on, in which at least oneof composition ratios of group III elements, band gap energy, refractiveindex, conductivity and specific resistance of nitride semiconductorlayers, which are formed on a base body having recess and contain aplurality of group III elements, varies within each layer in accordancewith recesses and projections of the base body to have one or both ofthe current confinement function and the optical waveguide function. Andhere again, by having nitride mixed crystal thin film layers on the basebody varied in physical properties within the horizontal plane of thebase body, which was difficult conventionally, it is possible to providea semiconductor device having a high yield, productivity necessary forcost reduction, excellent initial characteristics and reliabilitywithout using complicated processes.

Second Example

A specific example of the second embodiment explained above will be nextexplained below as the second example of the invention.

FIG. 6 is a diagram that shows a cross-sectional structure of a nitridesemiconductor laser according to the second example of the invention.That is, in the laser shown here, sequentially formed on a sapphiresubstrate 201 are a GaN buffer layer 202, Si-doped GaN n-type contactlayer 203, Si-doped AlGaN n-type etching stop layer 204, non-doped GaNetching layer 205, and Mg-doped AlGaN p-type etching mask layer 206. Astripe-shaped opening S is formed in the etching mask layer 206 and theetching layer 205, and a Si-doped GaN buried layer 207 is buried andformed flatly. The Mg-doped AlGaN etching mask layer 206 functions asthe current blocking layer.

Further formed on the Si-doped GaN buried layer 207 are a Si-doped AlGaNn-type cladding layer 208, Si-doped GaN n-type optical guide layer 209,InGaN multiquantum well (MQW) active layer 210, Mg-doped AlGaN p-typeblock layer 211, Mg-doped GaN p-type optical guide layer 212, Mg-dopedAlGaN p-type cladding layer 213, and Mg-doped GaN p-type contact layer214.

Then, those layers are selectively removed by etching from the p-typecontact layer 214 to a half depth of the n-type contact layer 203, ann-side electrode 216 is formed on the surface of the exposed n-typecontact layer 203, and a p-side electrode 215 is formed on the surfaceof the p-type contact layer 214.

Conventional ridge-structured lasers need a process of forming thep-side electrode 215 on the top surface of a ridge approximately 2 μmwide. This was a difficulty about reproducibility because of theprocess, and led to a decrease of the yield. In contrast, in the instantexample, since the etching mask layer 206 serving as the currentblocking layer is formed between the active layer 210 and the substrate201 and the surface is buried and flattened later, the p-side electrode215 can be made on the flat and wide surface.

Additionally, as already explained, ridge structures also involved theproblem that the difference in effective refractive index between theridge portion and the exterior of the ridge portion, important fortransverse mode control, largely depends on the etching depth.Therefore, ridge structures are seriously affected by controllability ofthe etching process and the film thickness profile, and this alsodecreases the yield. In contrast, the instant example is characterizedin being free from the etching depth control accuracy, having a highyield and ensuring stable device characteristics.

Next referring to FIG. 7, which shows steps of a process incross-sectional views, a manufacturing method of the nitridesemiconductor laser shown in FIG. 6 will explained below. One offeatures of the semiconductor laser manufacturing method of FIG. 7 liesin employing the aforementioned vapor phase etching to etch the etchinglayer 205 in the step of FIG. 7(c).

First with reference to FIG. 7(a), the buffer layer 202 composed of GaN,AlN or AlGaN is grown to a thickness around the range of 10 to 200 nm onthe sapphire substrate 201 by metal-organic chemical vapor deposition(MOCVD), and thereon, the Si-doped GaN n-type contact layer 203 is grownto the thickness of 3 μm. Subsequently, the Si-doped Al_(0.07)Ga_(0.93)Nn-type etching stop layer 204, 0.1 μm thick, non-doped GaN etching layer205, 0.2 μm thick, and Mg-doped Al_(0.2)Ga_(0.8)N p-type etching masklayer 206, 0.1 μm thick, are formed.

After that, the substrate is removed from the MOCVD apparatus, then asshown in FIG. 7(b), a resist or SiO₂ mask 800 is formed on the etchingmask layer 206, and a stripe-shaped opening O is formed in the mask 800by an exposure process, for example. Thereafter, using the opening O ofthe mask 800, the etching mask layer 206 is selectively removed byetching to obtain the stripe-shaped opening in the etching mask layer206 as shown in FIG. 7(b). In this etching process, dry etching or wetetching using KOH or other etching liquid may be employed. Thereafter,the mask 800 is removed, the substrate is again set in the MOCVDapparatus, 20 SLM of nitrogen gas is supplied, and the substratetemperature is raised to 1000° C. In this heating process, moisture andimpurity gas having adhered on the substrate are removed. In thenitrogen atmosphere, GaN and AlGaN are not etched substantially.

After that, introduction of hydrogen gas is started, and as shown inFIG. 7(c), the etching layer 205 exposed in the opening is etched by theaforementioned vapor phase etching. More specifically, when thesubstrate temperature reaches 1000° C. in the preceding heating process,introduction of hydrogen gas is started to flow a mixed gas of 20 SLM ofnitrogen gas and 10 SLM of hydrogen gas. Then the substrate is held inthe atmosphere at 1000° C. for two minutes. In this step, although theetching mask layer 206 and the etching stop layer 203 both made of AlGaNare not etched, the etching layer 205 made of GaN is etched. That is, inthis process, selective etching of GaN relative to GaN is attained.Therefore, as shown in FIG. 7(c), the GaN etching layer 205 is etched inthe depth direction, and the etching no more progresses in the depthdirection when it reaches the AlGaN etching stop layer 204.

Here is considered that the reason why GaN is etched lies in that itdecomposes and sublimates by reaction with hydrogen. For example, incase of GaN, by reacting with hydrogen, it presumably decomposes to Ga(vapor phase) and NH₃ (vapor phase), and results in being etched. On theother hand, it is considered that the reason why AlGaN is not etchedlies in that containment of Al enhances coupling force with group Velements and makes it difficult to decompose.

Experiments by the Inventor clarified that the etching temperature ispreferably maintained at an adequate value in order to selectively etchGaN relative to AlGaN. If the etching temperature is too high, AlGaN isalso etched in addition to GaN, and the etching rate increases too highto readily control the degree of side etching. In contrast, if theetching temperature is too low, the etching rate decreases to animpracticably low level. Through experiments by the Inventor, thetemperature range higher than 800° C. and not exceeding 1150° C., morepreferably, the temperature range from 1000° C. to below 1150° C., isconsidered appropriate. The instant example is an example in which thistemperature is set at 1000° C.

Furthermore, according to experiments by the Inventor, the mixture ratioof the hydrogen gas flow amount relative to the total gas flow amount ispreferably maintained in an appropriate value in order to selectivelyetch GaN relative to that AlGaN. If the mixture ratio of hydrogen isexcessively large, the etching rate becomes to high to readily controlthe side etching. In contrast, if the mixture ratio of hydrogen isexcessively small, the etching rate will become impracticably low.According to experiments by the Inventor, mixture ratio of the hydrogengas flow amount relative to the entire gas flow rate is preferably inthe range from 0.1 to 0.5. The instant example is an example in whichtotal gas flow amount is 30 SLM, hydrogen gas flow amount is 10 SLM, andthe mixture ratio is 0.33.

Additionally, according to experiments by the Inventor, aluminumcompositions of the etching stop layer 204 and the etching mask layer206 of FIG. 7(c) are desired to be 0.05 or higher. If the aluminumcomposition is below 0.05, the etching stop layer 204 and the etchingmask layer 206 are also etched, and the selectivity lowers. Although theinstant example uses GaN as the etching layer 205, subject to anappropriate selectivity being obtained, the etching layer 205 may bemade of AlGaN having a smaller aluminum composition than those of theetching stop layer 204 and the etching mask layer 206.

Next as shown in FIG. 7(d), layers are grown by crystal growth. First ofall, the supply of hydrogen gas is stopped, the supply of ammonium gasis started, and a mixed gas of 20 SLM of nitrogen gas and 10 SLM ofammonium gas is flown. Simultaneously, the substrate temperature israised to 1080° C. In this process, the temperature can be raised in themixed gas atmosphere containing nitrogen gas and ammonium gas whilepreventing the GaN etching layer 205 from being etched in the transversedirection. When the substrate temperature reaches 1080° C., the supplyof trimethyl gallium (TMG), silane gas and hydrogen gas issimultaneously started to grow the Si-doped GaN buried layer 207. Duringgrowth of this layer, the 0.3 μm deep opening S made in the foregoingetching process was buried flat by growing 0.2 μm thick Si-doped GaN.Further formed sequentially are the Si-doped AlGaN n-type cladding layer208, Si-doped GaN n-type optical guide layer 209, active layer of theInGaN multi quantum well (MQW) structure 210, Mg-doped AlGaN p-typeblock layer 211, Mg-doped GaN p-type optical guide layer 212, Mg-dopedAlGaN p-type cladding layer 213, and Mg-doped GaN p-type contact layer214.

Next as shown in FIG. 7(e), the multi-layered structure is etched toexpose the n-type contact layer 203. That is, the substrate is removedfrom the MOCVD apparatus, the mask 802 is formed on the p-type contactlayer 214 by an exposure process, and the layers are selectively etchedto a half depth of the n-type contact layer 203 by dry etching.

Next as shown in FIG. 7(f), the n-side electrode 216 is formed on theexposed surface of the n-type contact layer 203, and the p-sideelectrode 215 is formed on the surface of the p-type contact layer 214.Thereafter, the sapphire substrate is polished from its bottom surfaceuntil the thickness becomes 80 μm or less, and the structure is cleavedinto chips having laser edges. One-side laser edges undergohigh-reflectance coating of SiO₂ and TiO₂. Each chipped substrate ismounted on a heat sink and its n-side electrode and p-side electrodeunder wire bonding to complete a laser device.

As explained above, the instant example uses a series of selectiveetching process by vapor phase etching and re-growth as the method ofmaking the current confinement structure. As already explained,selective etching by vapor phase etching can stop the etching at theinterface between the GaN etching layer 205 and the AlGaN etching stoplayer 206. That is, even with a thickness profile along the surface ofthe grown etching layer 205, the GaN etching layer 205 is removed byetching to make out the current confinement structure.

In contrast, if the conventional dry etching is used for making thecurrent confinement structure, not yet having a sufficient selectiveetching condition, it is not possible to stop the etching at theinterface throughout the entire surface when a profile exists in filmthickness within the wafer surface, and it result in an unacceptableyield because the etching stop layer locally remain, or over-etched.There is also the problem that damage by dry etching is retained.

According to this example, the current confinement structure could bemade easily while ensuring an excellent reproducibility over a widerange of the substrate.

Third Example

Next explained is a modification of the foregoing second example, takenas the third example of the invention.

FIG. 8 is a schematic diagram that shows a cross-sectional structure ofthe semiconductor laser taken as the third example of the invention.Here are labeled common reference numerals to identical or equivalentcomponents to those shown in FIGS. 6 and 7, and their detailedexplanation is omitted.

In the foregoing second example, the non-doped GaN layer is used as theetching layer 205 as shown in FIG. 6. In the instant example, however,the etching layer 205 has a two-layered structure of a Si-doped ornon-doped GaN layer 205A and a Mg-doped GaN layer 205B as shown in FIG.8.

It would be radically desirable that the Mg-doped AlGaN layer 206serving as the current blocking layer has a higher Al composition and alarger film thickness. If actually so, however, cracks will be moreliable to occur, and in some cases, the layer 206 cannot be made thickenough to substantially function as the current blocking layer. In thisexample, therefore, Mg is doped into the GaN etching layer 205 to obtainan effective thickness as the current blocking layer.

However, if the Mg-doped GaN layer 205 alone is used as the etchinglayer, Mg will diffuse into the Si-doped AlGaN etching stop layer 204during the crystal growth process of Mg-doped GaN layer 205, and willform a p-type AlGaN layer along the interface between the n-type AlGaNetching stop layer 204 and the p-type GaN etching layer 205. And thep-type AlGaN layer will possibly behaves as a barrier between n-type GaN208 and n-type AlGaN layer 204. In the instant example, therefore,then-type GaN or non-doped GaN layer 205A is interposed between thep-type GaN etching layer 205B and the n-type AlGaN etching stop layer204. Employment of this structure enabled realization of the currentconfinement structure free from cracks inviting current leakage, evenwhen using the AlGaN etching mask layer (current blocking layer) 206with a high Al composition.

Fourth Example

Next explained is a fourth example of the invention, which is a specificexample using an InGaN etching layer 205C (FIG. 9) instead of the GaNetching layer 205 (FIGS. 6 through 8) used in the second and thirdexamples.

FIG. 9 is a schematic diagram that shows a cross-sectional structure ofa semiconductor laser taken as the fourth example. Here are used commonreference numerals for identical or equivalent components to those sownin FIGS. 6 through 8. In the semiconductor laser of FIG. 9, the etchinglayer 205C made of InGaN is provided. Further, omitting the AlGaNetching stop layer 204 shown in FIGS. 6 through 8, here is used theSi-doped GaN contact layer 203 as the etching stop layer. Further, hereis used Mg-doped GaN as the etching mask layer 206C in lieu of theMg-doped AlGaN etching mask layer 206 shown in FIGS. 6 through 8. Indiumcomposition of the InGaN etching layer 205C is preferably 0.3 or belowbecause In composition higher than 0.3 will deteriorate thecrystallographic property.

The semiconductor laser of FIG. 9 can be simplified in device structureby using the Si-doped GaN n-type contact layer 203 as the etching stoplayer.

In the semiconductor laser shown in FIG. 9, a stripe-shaped opening Scan be made by selectively etching the InGaN etching layer 205C relativeto the GaN etching mask layer 206C and the etching stop layer 203 bothmade of GaN substantially in the same manner as explained with referenceto FIG. 7(c). That is, since the InGaN etching layer 205C is more likelyetched than the etching mask layer 206C and the etching stop layer 203both made of GaN, selective etching is possible by using thatdifference. However, taking into account that the InGaN etching layer205C (FIG. 9) is more easily etched than the GaN etching layer 205 (FIG.7) and that the etching mask layer 206C and the etching stop layer 203(FIG. 9) both made of GaN are more likely etched than the etching masklayer 206 and the etching layer 204 (FIG. 7) both made of AlGaN, it isdesirable that the etching temperature is in the range from 600° C. to800° C., which is lower than the temperature range in the case of FIG.7(c). If the etching temperature is higher than 800° C., not only in theetching layer 205C but also in the etching mask layer 206C or even inthe etching stop layer 203, etching will progress undesirably. Incontrast, if the etching temperature is in the range not higher than600° C., the etching rate of the etching layer 205C will beimpracticably low.

The Inventor has confirmed through experiments that indium compositionof the etching layer 205C of FIG. 9 is desired to be 0.05 or more. Ifthe indium composition is below 0.05, etching rate of the etching layer205C will be excessively low, and selectivity between the etching stoplayer 203 and the etching mask layer 206 will be undesirably reduced.

In the laser device of FIG. 9, GaN is used as the etching stop layer 203and the etching mask layer 206. However, subject to an appropriateselectivity being obtained, InGaN having a lower indium composition thanthe etching layer 205C can be used instead. Preferably, however, indiumcomposition does not surpass 0.05 to ensure that the etching stop layer203 and the etching mask layer 206C are not etched.

Fifth Example

Next explained is a structure having voids at opposite side of theopening of the current confinement structure, as the fifth example ofthe invention.

FIG. 10 is a schematic diagram that shows a cross-sectional structure ofa semiconductor laser taken as the fifth example of the invention. Hereare used common reference numerals to identical or equivalent componentsto those shown in FIGS. 6 through 9, and their detailed explanation isomitted.

As shown in FIG. 10, the semiconductor laser as this example has voids(cavities) at opposite sides of the opening S of the current confinementstructure. These voids contributes to increasing the difference ineffective refractive index between inside and outside of the opening Sand facilitate fabrication of a laser oscillating in the basictransverse mode. That is, the contributes to enhancing the lightconfinement efficiency to the waveguide and improving the oscillationmode.

FIG. 11 presents schematic cross-sectional views illustrating amanufacturing process in a manufacturing method of the semiconductorlaser taken as the fifth example of the invention. Basic layer structureand manufacturing process are similar to those of the specific exampleof FIG. 7.

First as shown in FIG. 11(a), first crystal growth is conducted to growa buffer layer 202 composed of GaN, AlN or AlGaN, for example, to athickness around 10 through 200 nm on the sapphire substrate 201 bymetal-organic chemical vapor deposition (MOCVD), and thereon, theSi-doped GaN n-type contact layer 203 is grown to the thickness of 3 μm.Subsequently, the Si-doped Ga_(0.93)Al_(0.07)N n-type etching stop layer204, 0.1 μm thick, non-doped GaN etching layer 205, 0.2 μm thick, andMg-doped Ga_(0.8)Al_(0.2)N p-type etching mask layer 206, 0.1 μm thick,are formed.

Next as shown in FIG. 11(b), the etching mask layer 206 is etched. Morespecifically, the substrate is removed from the MOCVD apparatus, aresist or SiO₂ mask 800 having a stripe-shaped window is formed by anexposure process. Using this mask, the Mg-doped Al_(0.2)Ga_(0.8)N p-typeetching mask layer 206 is removed by etching to make a stripe-shapedopening O in the p-type etching mask layer 206. In this etching process,dry etching or wet etching using KOH or other etching liquid may beemployed.

Subsequently, as shown in FIG. 11(c), the etching layer 205 is etched tomake voids V.

More specifically, first removing the mask 800, the substrate is againset in the MOCVD apparatus. Then, 20 SLM of nitrogen gas is supplied andthe substrate temperature is raised to 1000° C. In this process, byheating it in the nitrogen gas atmosphere, moisture and impurity gashaving adhered on the substrate are removed. On the other hand, GaN andAlGaN are not etched. When the substrate temperature reached 1000° C.,introduction of hydrogen gas was started, and mixed gas of containing 20SLM of nitrogen gas and 10 SLM of hydrogen gas was supplied. In thisatmosphere, the substrate was haled at 1000 C for five minutes. Asalready explained, selective etching between AlGaN and GaN is attained.Although the Mg-doped Al_(0.2)Ga_(0.8)N p-type etching mask layer 206 isnot etched, the non-doped GaN etching layer 205 is etched.

Once the etching reaches the Si-doped Al_(0.07)Ga_(0.93)N n-type etchingstop layer 204, it no more progresses in the depth direction. However,by prolonging the etching time, side etching can be brought about inresponse to the time. In the instant example, vapor phase etching forfive minutes resulted in bringing about side etching of a width around 5μm.

Next as shown in FIG. 11(d), burying growth is conducted.

First all, the supply of hydrogen gas is stopped, the supply of ammoniumgas is started, and a mixed gas of 20 SLM of nitrogen gas and 10 SLM ofammonium gas is flown to prevent the vapor phase etching, and thesubstrate temperature is raised to 1080° C. When the substratetemperature reaches 1080° C., the supply of trimethyl gallium (TMG),silane gas and hydrogen gas is simultaneously started to grow theSi-doped GaN buried layer 207. As a result of growth of this layer, theside-etched portions, approximately 5 μm wide, remain as voids V in formof cavities, and the opening S of the current confinement structure isburied flat by growing 0.2 μm Si-doped GaN.

After that, similarly to the second example, sequentially formed are theSi-doped AlGaN n-type cladding layer 208, Si-doped GaN n-type opticalguide layer 209, active layer of the InGaN multi quantum well (MQW)structure 210, Mg-doped AlGaN p-type block layer 211, Mg-doped GaNp-type optical guide layer 212, Mg-doped AlGaN p-type cladding layer213, and Mg-doped GaN p-type contact layer 214.

Thereafter, the substrate is removed from the MOCVD apparatus, a mask802 is formed on the p-type contact layer 214 by an exposure process,and layers are selectively etches to a half depth of the n-type contactlayer 203 by dry etching as shown in FIG. 11(e).

Then, as shown in FIG. 11(f), the n-side electrode 216 was formed on theexposed surface of the n-type contact layer 203, and the p-sideelectrode 215 was formed on the surface of the p-type contact layer 214.

As explained above, according to the instant example, by adjusting timeor other conditions of the vapor phase etching to bring about sideetching of the etching layer 205 and thereafter forming the buried layer207, the voids V can be made reliably and easily at opposite sides ofthe opening S of the current confining portion. As a result, it ispossible to enhance the difference in effective refractive index betweeninside and outside of the current confinement structure, improve thelight confining efficiency of the waveguide and fabricate, easily with ahigh yield, the semiconductor laser oscillating in the basis transversemode.

Heretofore, some embodiments of the invention have been explained withreference to specific examples. The invention, however, is not limitedto these specific examples.

For example, a skilled in the art will be able to modify the devicestructures and materials of semiconductor lasers to which the inventionis applied, by choosing any known appropriate techniques and designs andwill be able to obtain substantially the same effects.

Further, the invention is applicable not only to semiconductor lasersbut also to light emitting diodes and other various optical devices andelectronic devices to obtain substantially the same effects.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1-25. (canceled)
 26. An etching method for selectively etching a firstnitride semiconductor layer relative to a second nitride semiconductorlayer comprising: etching said first nitride semiconductor layer byheating it in a mixed atmosphere containing hydrogen and at least one ofnitrogen, ammonium, helium, argon, xenon and neon; or a mixed atmosphereof nitrogen and ammonium; or and a hydrogen atmosphere.
 27. The etchingmethod according to claim 26 wherein aluminum composition of said secondnitride semiconductor layer is higher than aluminum composition of saidfirst nitride semiconductor layer.
 28. The etching method according toclaim 26 wherein indium composition of said second nitride semiconductorlayer is lower than indium composition of said first nitridesemiconductor layer.